In the study of electronic materials and processes for fabricating such materials into an electronic structure, a specimen of the electronic structure is frequently used for microscopic examination for purposes of failure analysis and device validation. For instance, a specimen of an electronic structure such as a silicon wafer is frequently analyzed in scanning electron microscope (SEM) and transmission electron microscope (TEM) to study a specific characteristic feature in the wafer. Such characteristic feature may include the circuit fabricated and the defect formed during the fabrication process. An electron microscope is one of the most useful equipment for analyzing the microscopic structure of semiconductor devices.
In preparing specimens of an electronic structure for electron microscopic examination, various polishing and milling processes can be used to section the structure until a specific characteristic feature is exposed. As device dimensions are continuously reduced to the sub-half-micron level, the techniques for preparing specimens for study in an electron microscope have become more important. The conventional methods for studying structures by an optical microscope cannot be used to study features in a modern electronic structure due to its unacceptable resolution.
In the FIB technique, focused ion beams can be used to either locally deposit or remove materials. When the cluster impacts the surface of an electronic structure, the cluster disintegrates into atoms which are then scattered over the surface to remove a surface layer of the material. Typical ion beams have a focused spot size of smaller than 100 nm when produced by a high intensity source. Sources of such high intensity ions can be either liquid metal ion sources or gas field ion sources. Both of these sources have a needle type form that relies on field ionization or evaporation to produce the ion beam. After the ion beam is produced, it is deflected in a high vacuum and directed to a desired surface area. The focused ion beams can be suitably used in the semiconductor processing industry in a cutting or attaching method to perform a circuit repair, a mask repair or a micromachining process. A cutting process is normally performed by locally sputtering a surface with a forced ion beam.
In an ion beam milling process, when a material is selectively etched by a beam of ions such as Ga.sup.+ focused to a sub-micron diameter, the technique is often referred to as focused ion beam etching or milling. FIB milling is a very useful technique for restructuring a pattern on a mask or an integrated circuit, and for diagnostic cross-sectioning of microstructures. In a typical FIB etching process, a beam of ions such as Ga.sup.+ is incident onto a surface to be etched and the beam can be deflected to produce a desirable pattern. The focused ion beam can be used to bombard a specimen surface such that a cavity can be formed on the surface of an electronic structure to review a characteristic feature of the structure for electron microscopic examination.
The FIB technique utilizes a primary beam of ions for removing a layer of material at a high current, and for observing the surface that was newly formed at a low current. The observation of the surface is made by detecting the secondary electrons emitted from the sample surface when the surface is bombarded by the ions. A detector is used to receive the secondary electrons emitted from the surface to form an image. The FIB method, even though can not produce an image of a high resolution like that obtainable in a SEM/TEM, can be used to sufficiently identify a newly formed cross-sectional surface which may contain the characteristic feature to be examined. The capability of the FIB technique for making observations down to a resolution of 5.about.10 nm enables the cutting of a precise plane in an electronic structure such that it may be later examined by a SEM or TEM technique at a higher resolution than that capable with FIB.
FIGS. 1A through 1E illustrate a conventional process of preparing a SEM/TEM sample of an electronic structure which is first cut precisely by FIB for electron microscope observation. Referring initially to FIG. 1A, wherein an electronic structure 10 is shown. The electronic structure 10 has a top surface 12 and a bottom surface 14 on a silicon substrate 16. On the top surface 10, a cavity 18 is formed by a focused ion beam which has sidewalls 20 and 22 formed at an angle of approximately 90.degree. and 45.degree., respectively with the top surface 12. The FIB technique reveals a characteristic feature, i.e., a defect, located on the sidewall 20. A layer of adhesive 24 is then applied on the top surface 12 of the structure 10. This is shown in FIG. 1B.
It should be noted that when the adhesive layer 24 is applied, the adhesive not only on the top surface 12 but also covers fills up the cavity 18. In the next step of the process, a glass slide 28 is used to cover the top surface 12 of electronic structure 10 bonded by the adhesive layer 24. The function of the glass slide 28 is to protect the top surface 12 of the electronic structure 10 such that during a subsequent grinding process, the top surface 12 does not delaminate and thus destroying the cavity 18 and the characteristic feature situated on the sidewall 20. The glass/glue/electronic structure laminate is then mechanically polished by a polishing disc or a grinding wheel in a plane perpendicular to the planar surface of the electronic structure 10. The grinding process can be controlled by a periodic observation through the glass slide 28 in an optical microscope. During such observation, the sample can be tilted 90.degree. to reveal a newly formed cross-section by the polishing process in order to determine the end point of the polishing process.
As shown in FIG. 1D, the polishing process is stopped when the electronic structure 10 is polished to reveal the sidewall 20 covered by a glue layer 24. In order to observe the characteristic feature on the sidewall 20, the glue layer 24 that covers the sidewall 20 must be removed. As shown in FIG. 1E, the removal process frequently damages the sidewall 20 by removing the characteristic feature together with the glue layer 24 and forming a new cross-section 28 without the characteristic feature.
The conventional process of preparing a SEM/TEM sample which is cut precisely by FIB first for microscopic observation by SEM/TEM, as shown in FIGS. 1A.about.1E, is therefore inadequate in providing a reliable sample surface. The glue layer 24 shown in FIGS. 1B.about.1E is necessary for bonding a glass slide 28 to the electronic structure 10. However, the glue layer inevitably fills up the cavity created by FIB and consequently, destroys the characteristic feature during a glue removal process.
It is therefore an object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination during which a glass slide must be bonded to a sample surface that does not have the drawbacks or shortcomings of the conventional sample preparation methods.
It is another object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination that requires the bonding of a glass slide to an electronic structure by an adhesive layer wherein a subsequent adhesive removal step does not affect the characteristic feature to be examined.
It is a further object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination that requires a glass slide to be bonded to a substrate by an adhesive layer wherein the adhesive layer is prevented from entering into a cavity that was previously formed and contains a characteristic feature to be examined.
It is another further object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination that requires a glass slide to be bonded to an electronic structure by an adhesive layer wherein a cavity prepared by FIB in a top surface of the structure is first filled by a material that can be subsequently removed by a solvent to prevent the intrusion of the adhesive into the cavity.
It is still another object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination that requires a glass slide to be bonded to an electronic structure by an adhesive layer wherein a cavity formed in the top surface of the substrate is first filled by a wax-based material prior to the application of the adhesive layer.
It is yet another object of the present invention to provide a method for preparing samples which is cut precisely by FIB first for microscopic examination that requires a glass slide to be bonded to an electronic structure by an adhesive layer wherein a cavity formed in the top surface of the substrate is first filled by a wax-based material prior to the bonding of the glass substrate and the wax-based material is subsequently removed by a hydrocarbon solvent prior to the electron microscopic examination.